Antifouling paints obtained by blending vehicle components such as a toughener, a rosin, a plastisizer, etc. with an antifouling agent, or antifouling paints using polymers or copolymers of organotin-containing unsaturated monomers as described in, e.g., Japanese Patent Publication Nos. 21,426/1965, 9,579/1969 and 12,049/1976 have been generally coated so far on underwater portions of ships or marine structures to prevent corrosion due to adhesion of submarine organisms such as barnacle, ascidian, seaweed, etc. and on equipment such as a cultivation net for useful ocean culture to prevent death of fish and shellfish owing to adhesion of submarine organisms.
The former antifouling paints exhibit an antifouling effect by dissolving the rosin and the antifouling agent contained in the coated film obtained therefrom. However, when amounts of insoluble matters become large in the coated film during long-term dipping in seawater, the coated surface becomes uneven, resulting in decrease of an effect to prevent adhesion of submarine organisms. Meanwhile, the latter antifouling paints contain the organotin compound introduced into the (co)polymers via an ester linkage. When the coated film formed therefrom is dipped in seawater, the organotin compound is released and a carboxyl group-containing polymer is formed by hydrolysis of the ester linkage. This polymer is dissolved in seawater and a fresh coated film is always exposed, maintaining the antifouling property. Nevertheless, many organotin compounds are ordinarily introduced into the antifouling paints to effectively exhibit the antifouling property of the coated film, and give an adverse effect on fish and shellfish.
For improving the aforesaid defects, a method in which an antifouling coated film is formed using an antifouling paint containing an acid anhydride group-containing resin as a vehicle component and an antifouling agent such as cuprous oxide is proposed in Japanese Laid-open Patent Application No. 99,567/1990. The resin used in this method is a copolymer having a cyclic acid anhydride group in a molecule and obtained by subjecting a polymerizable unsaturated acid anhydride such as itaconic anhydride or maleic anhydride to radical copolymerization reaction with another unsaturated monomer. A coated film formed by using a paint containing said copolymer as a vehicle component, upon hydrolysis, forms two carboxyl groups from the acid anhydride to render the copolymer water-soluble. As soon as the copolymer is dissolved in seawater, the antifouling agent present, such as cuprous oxide, is also dissolved therein, and a fresh coated film is always exposed, maintaining anti-fouling property. Since, however, the hydrolysis which the acid anhydride group of the copolymer undergoes by a moisture of air or seawater proceeds very quickly, the coated film formed by the above method can hardly keep a good antifouling property for a long period of time.
It is an object of this invention to provide an antifouling paint composition which maintains excellent antifouling property for a long period of time by controlling a rate of hydrolysis of a resin as a vehicle component, and which can form a less toxic coated film.
The present inventors have made extensive investigations to achieve the object, and as a result, have found that a rate of hydrolysis of a resin as a vehicle component can be controlled by containing in the vehicle resin component a nonaqueous dispersion-type resin of a skin-core structure having an acid group in the core.